Hazardous metals are a significant human health problem in heavily industrialized urban areas worldwide. Unfortunately, human pathologies often appear only after prolonged exposure, and delayed detection is costly both financially and in terms of quality of life. Furthermore. direct monitoring of metals in humans (e.g., through blood samples) is logistically complex and intrusive, making it difficult to identify populations at risk. 'Sentinel' animal species have received considerable recent attention as attractive alternatives for pollution monitoring. For use as a monitor, 'sentinels' must exhibit a predictable relationship between organism health and environmental quality. The health determinant should be reproducible and easily measured. Subtle deviations from bilateral symmetry (e.g., fluctuating asymmetries) offer such a measure. In both laboratory and field studies of a wide range of stressors, the average deviation from symmetry in organisms increases with increased stress levels (i.e., the development of perfect bilateral symmetry is disrupted). Fluctuating asymmetry in terrestrial isopods offers a new and potentially widely-applicable system for monitoring hazardous metals in urban environments. These detritivorous crustaceans are abundant in urban and rural areas worldwide, are easily collected by active or passive sampling, and accumulate high levels of metals (including copper, zinc, cadmium, and lead), which they retain for their entire lives. A pilot study of the numbers of compound eye 'lenses' (ommatidia) in the right and left eyes of the isopod Armadillidium vulgare revealed that asymmetry in ommatidia number was significantly greater in isopods collected from metal-contaminated sites in south Chicago, compared to isopods in less industrialized areas. Average asymmetry increased in a nearly linear dose- response relationship with total concentrations of beryllium, chromium, manganese, iron, copper, zinc, cadmium, barium, mercury, and lead in the animals (r2=0.89). This proposal is to evaluate fluctuation asymmetry in isopods as a measure of hazardous metal contamination in six urban neighborhoods in the Lake Calumet district of south Chicago, Illinois. Residential areas in this region are surrounded by numerous sources of metal pollution, and the majority are constructed atop metal-contaminated fill material, including: steel industry and industrial waste, coal/coke ash, and sludge. If successful, our approach could prove extremely valuable as a convenient, inexpensive and widely-applicable early warning system for identifying long-term threats to human health from toxic metals.